1. Field of the Invention
The present invention concerns a diaphragm device for an x-ray apparatus that is provided for scanning a subject, and a method for scanning a subject with such a diaphragm device.
2. Description of the Prior Art
A diaphragm device is known from DE 102 42 920 A1 with which the radiation beam can be adjusted in a very precise manner on the measurement field of the detector to avoid an unnecessary radiation exposure given a scanning of a subject with an x-ray apparatus, for example in the form of a computed tomography apparatus. To adjust the radiation beam emanating from the x-ray radiator, the diaphragm device has a diaphragm with two beam-proximate absorber elements. The diaphragm is fashioned such that both absorber elements can be positioned (set) with a high adjustment precision before the beginning of an examination.
Given a helical scan of a subject in which the acquisition system rotates around a system axis of the computed tomography apparatus and with the subject being simultaneously displaced relative to the acquisition system in the direction of the system axis, for reconstruction of an image within a usable volume it is necessary to irradiate a scan volume that is larger in the direction of the system axis than the usable volume. The larger (in comparison to the usable volume) scan volume significantly depends on the algorithm used for reconstruction of the image. the total irradiated volume results from usable volume itself (defined by the reconstruction algorithm that will be used) plus a number of additionally-required rotations, or the additionally required fraction of a rotation that must be implemented during a leading or advance movement and a trailing movement of the x-ray radiator. Only a fraction of the information acquired during the leading movement and trailing movement, however is used later for reconstruction, such that the subject is exposed to an unnecessary radiation exposure during these segments of the scan.
With increasing volume coverage of the detectors in the direction of the system axis, the number of the rotations of the acquisition system for scanning the usable volume can in fact be reduced, but the number of the rotations of a spiral scan during the leading and trailing movements that are necessary for complete reconstruction of the usable volume remain unaffected by the volume coverage of the detector. As a consequence, the proportion of the rotations due to the leading and the trailing movement thus increases in comparison to the rotations that are required for the total scanning. The dose efficiency, thus the actual proportion of the radiation used for reconstruction, is thus simultaneously reduced.
For example, in a computed tomography apparatus having a detector with Z=16 lines and a line width of B=0.75 mm and an x-ray radiator that radiates the 12 mm-wide measurement field in the direction of the system axis of the computed tomography apparatus with a radiation beam adjusted (gated) by a diaphragm device, the proportion of the scan due to the leading -and the trailing movement in comparison to the total scan of the sample volume amounts to 12%, given a usable volume to be scanned of L=200 mm, a set pitch of P=1 and given one full rotation of the acquisition system required for reconstruction during each of the leading movement and the trailing movement of the scan. Only approximately half of the radiation applied during the leading and trailing movement contributes to the reconstruction of an image, such that the radiation exposure of the patient is approximately 6% of the radiation dose applied in total.
Given the use of a detector with Z=128 lines and a line width of B=0.6 mm, the proportion of the leading movement and the trailing movement in the entire scan would increase to approximately 77%, such that the additional radiation exposure of the patient increases to approximately 39% of the radiation dose applied overall.